Dark tourism - the tourism of sites of tragedy - may be a recent growth area for the travel industry but it's not a new phenomenon. As far back as the Dark Ages, pilgrims were travelling to tombs and sites of religious martyrdom. The Battle of Waterloo in 1815 was observed by nobility from a safe distance and one of the earliest battlefields of the American Civil War (Manassas) was sold the next day as a visitor attraction site. More recently, Ground Zero in New York has become an essential part of the tourist itinerary for many visitors.

But why are these sites so popular? Our motivations are murky and difficult to unravel: a mix of reverence, voyeurism and maybe even the thrill of coming into close proximity with death.

The difference between what is acceptable and unacceptable as a tourist attraction is often only a matter of chronological distance. Walking tours of Jack the Ripper's London are enduringly popular. The world he inhabited is distant enough from our own for his exploits to be deemed entertaining. A Yorkshire Ripper trail would be seen as highly inappropriate by most people.

Over half a million people visit Auschwitz Birkenau each year. The standard two-and-a-half-hour guided tour gives an audience at best an abridged understanding of this vast and sprawling site. What a tourist can comprehend in such a brief visit is questionable.

Yet the operators of these diverse sites have become the custodians of history, and this carries with it certain responsibilities. Which parts of history do you choose to interpret and commemorate? Where, for example, are the monuments and visitor centres to the Roma and Sinti Holocaust and the genocide of the American Indians?

The coverage is often selective, motivated by the need to increase visitation and revenue rather than presenting an undistorted view of history. 'Dark tourism' sites are important testaments to the consistent failure of humanity to temper our worst excesses and, managed well, they can help us to learn from the darkest elements of our past. But we have to guard against the voyeuristic and exploitative streak that is evident at so many of them.

· Professor John Lennon is co-author of Dark Tourism - The attraction of death and disaster(Continuum) and director of the Moffat Centre for Travel and Tourism at Glasgow Caledonian University.